How to eliminate black stripe on the horizon from HDRI textures?
If you are learning to use HDR, use a proper HDR image that is meant to be used for a 3D rendering. 

You need an image that covers a range of 360 degrees preferably in equirectangular projection or Quadrilateralized spherical cube



In other words: a panoramic image that you can use as the world. Think of it as a large sphere that surrounds and lights your scene. It has to contain the information of the ground and the sky all around.

See: http://blog.gregzaal.com/tag/equirectangular/